CFD Analysis of Artificially Roughened Solar Air Heater

Abstract

Solar air heaters have low thermal efficiency because of low convective heat transfer coefficient between the air and the absorber plate which leads to higher temperature of the absorber plate and causes maximum thermal losses. The main thermal resistance to heat transfer from absorber plate to the flowing air is formation of laminar sub layer on the heat transferring surface. It is recommended to break this laminar sub layer in order to augment the heat transfer at the surface. In the present study, CFD Analysis of Solar air heater duct is carried out without and with artificial roughness. The effects of various roughness geometries viz. semi-circular, rectangular, triangular, combination of quarter circle, square and triangle, NACA 0030 profile etc. were studied in the wide range of Reynolds number between 6000 to 18000. Different turbulence models were used for the analysis viz. RNG k-epsilon, standard k- epsilon, realizable k-epsilon, SST k- omega, and the results are compared with analytical results. Among various models, RNG k-epsilon model is found to be most appropriate. Different roughness geometries led to 1.15 to 1.8 times increase in Nusselt number and 2.7 to 4.4 times increase in friction factor. As Among, various roughness geometries, the results obtained with NACA 0030 profile were found to be optimum in context of heat transfer and pressure drop.